It is well-known that moisture is deeply involved in various products such as cosmetics, pharmaceuticals, agricultural chemicals and foods, and moisturization thereof is one of important functions for the quality of the products. It is also well-known that moisture is deeply involved in the retention of youthful skin, and the moisturization of skin is one of important functions for the quality of cosmetics.
As known moisturizing agents, particularly known moisturizing agents used in cosmetics or external preparations, there are polyhydric alcohols such as glycerol, 1,3-butylene glycol and sorbitol, and further pyrrolidonecarboxylate salts, lactate salts, etc. In recent years, sodium hyaluronate produced by a microorganism has come to be used.
Moisturizing agents also play an important role to work as an agent retaining the moisture of cosmetics or external preparations themselves and thereby contribute to the retention of the stability of the system. Trihydric or more polyhydric alcohols such as glycerol and sorbitol have excellent moisturizing properties and moisture retainability, and further in view of their safety, stability, prices, etc., are used as most general purpose moisturizing agents.
As to requirements which moisturizing agents should have, it is desirable that they are not influenced by environmental conditions (temperature, humidity, wind, etc.), and particularly can retain moisture for a long time regardless of the surrounding humidity. However, in the case of any moisturizing agent, the amounts of the absorption and release of moisture are influenced by the surrounding humidity because of its vapor pressure. In the case of glycerol, for instance, its equilibrium moisture content at 25.degree. C. is 60% when the relative humidity is 75%, and 15% when the relative humidity is 33%, and in the case of sorbitol, its equilibrium moisture content at 25.degree. C. is 50% when the relative humidity is 75%, and 5% when the relative humidity is 33%, and their moisture contents are influenced by the degree of the surrounding relative humidity.
Thus, there is a problem that when the surrounding humidity is low, the moisturizing agent itself releases the retaining moisture and the amount of moisture retained is lowered. Also when cosmetics or external preparations containing moisturizing agents are used, as understood from the above, moisture is lost from the skin with time lapse under low humidities, and the moisturizing function is lowered. Therefore, as to moisturizing agents, it is necessary that the evapotranspiration of moisture under low humidity environments is slow.